Winding type battery cell and lithium ion secondary battery

Abstract

The invention relates to the field of lithium ion batteries, and discloses a winding type battery cell and a lithium ion secondary battery. The winding type battery cell comprises a positive plate and electrolyte, the positive plate comprises a positive current collector and a positive active layer, a bottom coating and a surface coating are arranged at the winding tail end of the positive plate, the bottom coating is located between the positive current collector and the positive active layer, and the bottom coating comprises first inorganic particles; the surface coating layer is located on the surface, deviating from the positive electrode current collector, of the positive electrode active layer, and the surface coating layer comprises second inorganic particles; the electrolyte comprises a carboxylic ester solvent, and the mass content of the carboxylic ester solvent is 10%-50% by taking the mass of the electrolyte as a reference. By arranging the bottom coating and the surface coating at the winding tail end of the positive plate, the safety performance of the battery cell can be effectively improved on the premise that the energy density of the battery cell is hardly lost, and the low-temperature performance of the battery cell can be improved due to the existence of a proper amount of carboxylic ester solvent in the electrolyte.

Description

Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, and particularly relates to a wound cell and a lithium-ion secondary battery. Background Art

[0002] In recent years, lithium-ion secondary batteries have been widely used in daily life. For example, they can be found in electronic products such as smart phones and laptop computers, as well as in transportation means such as electric bicycles and electric vehicles. As the capacity of a single lithium-ion secondary battery cell becomes larger and larger, higher requirements are also put forward for the safety performance of lithium-ion secondary batteries.

[0003] At present, the safety performance tests for lithium-ion secondary batteries mainly include mechanical abuse represented by needle puncture and thermal abuse represented by hot box tests. To improve the passing rate of the battery in the above tests, related technologies have disclosed setting a bottom coating on the positive / negative current collector to reduce the possibility of the battery catching fire, exploding, or setting a surface coating on the surface of the positive / negative active paste to improve the thermal stability of the battery. Such batteries with good thermal safety usually use a high-temperature-resistant electrolyte system, which will result in poor low-temperature performance of the battery and cannot meet the usage requirements in low-temperature environments. Summary of the Invention

[0004] In view of this, the present application provides a wound cell, aiming to solve the problem of poor low-temperature performance of existing cells.

[0005] According to an embodiment of the present application, in a first aspect, a wound cell is provided, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet, the separator, and the negative electrode sheet are stacked and wound; the positive electrode sheet includes a positive current collector and a positive active layer provided on at least one surface of the positive current collector. A bottom coating and a surface coating are provided at the winding end of the positive electrode sheet;

[0006] The bottom coating is located between the positive current collector and the positive active layer, and the bottom coating includes first inorganic particles;

[0007] The surface coating is located on the surface of the positive active layer facing away from the positive current collector, and the surface coating includes second inorganic particles;

[0008] The electrolyte includes a carboxylic ester solvent, and the content of the carboxylic ester solvent is A% based on the mass of the electrolyte, where 10 ≤ A ≤ 50.

[0009] In some alternative embodiments, the positive electrode sheet includes a first electrode region, a second electrode region, and a third electrode region, which are arranged in sequence along the winding direction of the positive electrode sheet; the first electrode region includes the positive electrode current collector and the bottom coating, the positive electrode active layer, and the surface coating disposed on the surface of the positive electrode current collector facing the inner side of the wound battery cell; the resistivity of the first electrode region is R, and 200 Ω·cm ≤ R ≤ 1000 Ω·cm.

[0010] In some alternative embodiments, it satisfies: 500 Ω·cm ≤ R ≤ 800 Ω·cm.

[0011] In some alternative embodiments, it satisfies: 8 ≤ R / A ≤ 80.

[0012] Further, it satisfies: 10 ≤ R / A ≤ 80.

[0013] In some alternative embodiments, the carboxylic ester solvent includes at least one of methyl formate, ethyl acetate, ethyl propionate, methyl acetate, methyl butyrate, methyl propyl carbonate, propyl propionate, ethyl butyrate, methyl fluoroformate, ethyl difluoroacetate, ethyl 2-fluoropropionate, methyl fluoroacetate, methyl 4-fluorobutyrate, propyl 2-fluoropropionate, and ethyl 4-fluorobutyrate.

[0014] In some alternative embodiments, the electrolyte further includes a lithium salt and a nitrile additive, and the content of the nitrile additive is B% based on the mass of the electrolyte, and 2 ≤ B ≤ 10.

[0015] Further, it satisfies 30 ≤ R / B ≤ 200.

[0016] In some alternative embodiments, the nitrile additive includes at least one of succinonitrile, glutaronitrile, adiponitrile, trans-butenedinitrile, trans-hexenedinitrile, glycerol trinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile, 1,2,3-tris(cyanoethoxy)propane, and 1,4-dicyano-2-butene.

[0017] In some alternative embodiments, the second electrode region includes the positive electrode current collector and the bottom coating and the positive electrode active layer disposed on both surfaces of the positive electrode current collector;

[0018] The third electrode region includes the positive electrode current collector and the positive electrode active layer disposed on both surfaces of the positive electrode current collector;

[0019] In the ARC self-heating test, the time required for the surface temperature of the first electrode region to reach 150 °C is T 1The time required for the surface temperature of the second electrode sheet area and / or the third electrode sheet area to reach 150 °C is T 2 , T 1 -T 2 > 300 min.

[0020] In some alternative embodiments, the coating porosity P of the first electrode sheet area 1 is 10%-14%, the coating porosity P of the second electrode sheet area 2 is 14%-18%, the coating porosity P of the third electrode sheet area 3 is 18%-25%, satisfying P 1 < P 2 < P 3 .

[0021] In some alternative embodiments, on the same side in the thickness direction of the positive electrode sheet, the length L of the bottom coating 1 and the length L of the positive electrode active layer 2 and the length L of the surface coating 3 satisfy: (L 1 + L 3 ) ≤ 1.5 * L 2 .

[0022] Further, the L 1 is 100 mm - 2000 mm.

[0023] Further, the L 2 is 500 mm - 3000 mm.

[0024] Further, the L 3 is 50 mm - 500 mm.

[0025] In some alternative embodiments, the surface coating and / or the bottom coating are partially embedded in the positive electrode active layer, and the depth h of the surface coating embedded in the positive electrode active layer 1 is less than the depth h of the bottom coating embedded in the positive electrode active layer 2 .

[0026] Further, the depth h of the surface coating embedded in the positive electrode active layer 1 is 0.5 μm - 3 μm.

[0027] Further, the depth h of the bottom coating embedded in the positive electrode active layer 2 is 1 μm - 3 μm.

[0028] In some alternative embodiments, in the thickness direction of the positive electrode sheet, the minimum thickness h of the surface coating 3is 0.5 μm - 4 μm, and the minimum thickness h of the bottom coating 4 is 2 μm - 6 μm, satisfying h 3 ≤ h 4 .

[0029] In some alternative embodiments, based on the mass of the bottom coating, the bottom coating comprises 80% - 99% of first inorganic particles, 0.5% - 10% of a first binder, and 0.5% - 10% of a conductive agent.

[0030] In some alternative embodiments, based on the mass of the surface coating, the surface coating comprises 90% - 99% of second inorganic particles and 1% - 10% of a second binder.

[0031] In some alternative embodiments, the first inorganic particles and / or the second inorganic particles comprise at least one of silica, magnesia, alumina, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide.

[0032] In some alternative embodiments, the first binder and / or the second binder comprise at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide, or their derivatives.

[0033] In some alternative embodiments, the conductive agent comprises at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, metal powder, carbon fiber, and graphene.

[0034] Furthermore, the metal powder comprises at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, and aluminum powder.

[0035] In some alternative embodiments, the separator comprises a substrate, a first adhesive layer, a ceramic layer, and a second adhesive layer. The first adhesive layer is disposed on one surface of the substrate, the second adhesive layer is disposed on the surface of the substrate facing away from the first adhesive layer, and the ceramic layer is located between the substrate and the second adhesive layer and faces the positive electrode sheet;

[0036] The ceramic layer comprises ceramic particles, and the median particle size Dv50 of the ceramic particles and the average thickness H of the surface coating satisfy: 0.5 ≤ H / Dv50 ≤ 20, where H is 1 μm - 10 μm and Dv50 is 0.5 μm - 2 μm.

[0037] In some alternative embodiments, the ceramic particles comprise at least one of boehmite, alumina, zirconia, silica, silicon carbide, and silicon nitride.

[0038] According to an embodiment of the present application, in a second aspect, a lithium-ion secondary battery is provided, which includes the wound core described in the first aspect of the present application.

[0039] The technical solution of the present application has the following advantages:

[0040] For the wound core provided by the present application, a bottom coating and a surface coating are provided at the winding end of the positive electrode sheet. The bottom coating includes first inorganic particles, and the surface coating includes second inorganic particles. The bottom coating can prevent contact short circuit between the positive electrode current collector and the negative electrode active material layer, and the surface coating can isolate the contact between the positive electrode sheet outside the core and the electrolyte, reducing heat generation from side reactions, thereby effectively improving the overall safety of the core. At the same time, an appropriate amount of carboxylic ester solvent is added to the electrolyte, which can improve the low-temperature performance of the core, so that the wound core of the present application has both good safety and low-temperature performance.

[0041] Additional aspects and advantages of the embodiments of the present application will be described and shown in part in the following description, or will be explained through the implementation of the embodiments of the present application. Description of the Drawings

[0042] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for use in the description of the specific embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0043] Figure 1 It is a schematic cross-sectional structure diagram of a wound core in an embodiment of the present application.

[0044] Figure 2 It is a schematic cross-sectional structure diagram of a positive electrode sheet in the length direction in an embodiment of the present application.

[0045] Figure 3 It is a cross-sectional SEM diagram of the surface coating in an embodiment of the present application.

[0046] Figure 4 It is a cross-sectional SEM diagram of the bottom coating in an embodiment of the present application.

[0047] Among them, the description of the reference numerals is as follows:

[0048] 11. Positive electrode current collector; 12. Positive electrode active layer; 13. Bottom coating; 14. Surface coating; 15. Positive electrode tab; 21. Negative electrode current collector; 22. Negative electrode active layer; 23. Negative electrode tab; 30. Separator; 31. Termination glue;

[0049] I. First electrode region; II. Second electrode region; III. Third electrode region;

[0050] L 1 is the length of the bottom coating; L 2 is the length of the positive electrode active layer; L 3 is the length of the surface coating. Detailed implementation manners

[0051] The following embodiments are provided to better understand the present application further. They are not limited to the described best implementation manners, and do not limit the content and protection scope of the present application. Any product that is the same as or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with those of other prior arts falls within the protection scope of the present application.

[0052] In the description of the present application, it should be noted that the orientation or positional relationship indicated by terms such as "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be understood as a limitation to the present application. The terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0053] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0054] To improve the low-temperature performance of the battery cell, according to the first aspect of the present application, a wound battery cell is provided. Refer to Figure 1 , the wound battery cell includes a positive electrode sheet, a negative electrode sheet, a separator 30 and an electrolyte. The positive electrode sheet, the separator 30 and the negative electrode sheet are stacked and wound; a plurality of positive electrode tabs 15 are provided on the positive electrode sheet located in the flat area of the wound battery cell, and a plurality of negative electrode tabs 23 are provided on the negative electrode sheet located in the flat area of the wound battery cell; meanwhile, a termination adhesive 31 is further provided at the winding end of the positive electrode sheet.

[0055] Combined with Figure 2 , the positive electrode sheet includes a positive electrode current collector 11 and a positive electrode active layer 12 provided on at least one surface of the positive electrode current collector 11; the direction indicated by the arrow is the winding direction of the positive electrode sheet. A bottom coating 13 and a surface coating 14 are provided at the winding end of the positive electrode sheet. The bottom coating 13 is located between the positive electrode current collector 11 and the positive electrode active layer 12, and the bottom coating includes first inorganic particles; the surface coating 14 is located on the surface of the positive electrode active layer 12 facing away from the positive electrode current collector 11, and the surface coating includes second inorganic particles;

[0056] The electrolyte includes a carboxylate solvent. Based on the mass of the electrolyte, the mass content of the carboxylate solvent is A%, where 10 ≤ A ≤ 50.

[0057] In the mechanical abuse tests such as needle puncture of a wound-type battery cell, the closer to the outer side of the battery cell, the greater the deformation amplitude caused by the abuse, and the easier it is to trigger an internal short circuit, especially the contact short circuit between the positive current collector and the negative active material. Since the heat generation power is the highest, it is the most dangerous. Therefore, in the present application, a bottom coating is provided at the winding end of the positive electrode sheet (corresponding to the outer side of the wound-type battery cell), which can prevent the contact short circuit between the positive current collector and the negative active material layer and improve the passing rate of the battery cell in the needle puncture test.

[0058] In the hot box test, the outer side of the wound-type battery cell is heated fastest and is also most likely to cause thermal runaway. Therefore, in the present application, a surface coating is provided at the winding end of the positive electrode sheet, which can isolate the contact between the positive electrode sheet on the outer side of the battery cell and the electrolyte, reduce the heat generation from side reactions, and thus improve the thermal safety of the battery cell.

[0059] In the present application, only by providing a bottom coating and a surface coating at the winding end of the positive electrode sheet can the risk of the battery cell in tests such as needle puncture and hot box be effectively reduced, the overall safety performance of the battery cell be improved, and at the same time, it can prevent the battery cell from being too thick and affecting the volume energy density, ensuring the improvement of its overall safety without affecting the electrical performance of the battery cell.

[0060] It can be understood that to ensure the safety performance of the battery, a high-temperature-resistant electrolyte system is usually required to be used in combination. However, this will result in poor low-temperature performance of the battery and cannot meet the usage requirements in a low-temperature environment. Therefore, in the present application, by adding an appropriate amount of carboxylate solvent to the electrolyte, the low-temperature performance of the battery cell can be improved, so that the wound-type battery cell of the present application has both good safety performance and low-temperature performance.

[0061] As an example, the mass content A% of the carboxylate solvent in the electrolyte can be 10%, 20%, 30%, 40%, 50%, etc. or within the range composed of any two of the above values. The present application has found that if the content of the carboxylate solvent in the electrolyte is too small, the improvement effect on the low-temperature performance of the battery is not obvious. On the contrary, if the content of the carboxylate solvent in the electrolyte is too large, it will affect the thermal safety of the battery.

[0062] As an example, the carboxylate solvent includes at least one of methyl formate, ethyl acetate, ethyl propionate, methyl acetate, methyl butyrate, methyl propyl carbonate, propyl propionate, ethyl butyrate, methyl fluoroformate, ethyl difluoroacetate, ethyl 2-fluoropropionate, methyl fluoroacetate, methyl 4-fluorobutyrate, propyl 2-fluoropropionate, and ethyl 4-fluorobutyrate.

[0063] It should be noted that the outer side of the battery cell refers to the side away from the winding center of the wound battery cell with the winding center of the wound battery cell as a reference. Correspondingly, the inner side of the battery cell refers to the side toward the winding center with the winding center of the wound battery cell as a reference.

[0064] Please continue to see Figure 2 , the direction indicated by the arrow is the winding direction (i.e., the length direction) of the positive electrode sheet, and the positive electrode sheet includes the third electrode sheet area III, the second electrode sheet area II, and the first electrode sheet area I in sequence along the winding direction. Among them, the first electrode sheet area I includes the positive electrode collector 11 and the bottom coating 13, the positive electrode active layer 12, and the top coating 14 arranged on the surface of the positive electrode collector 11 facing the inner side of the wound battery cell, the second electrode sheet area II includes the positive electrode collector 11 and the bottom coating 13 and the positive electrode active layer 12 arranged on the two side surfaces of the positive electrode collector 11, and the third electrode sheet area III includes the positive electrode collector 11 and the positive electrode active layer 12 arranged on the two side surfaces of the positive electrode collector 11. In addition, the winding tail end of the positive electrode sheet also includes an empty foil area, that is, no coating is provided on the positive electrode collector in this area.

[0065] In some embodiments, the resistivity R of the first pole piece region is in the range of 200Ω·cm-1000Ω·cm. As an example, R may be 200Ω·cm, 300Ω·cm, 400Ω·cm, 500Ω·cm, 600Ω·cm, 700Ω·cm, 800Ω·cm, 900Ω·cm, 1000Ω·cm, etc. or in the range of any two of the above values. Compared with the positive electrode active layer, both the undercoat layer and the topcoat layer have lower conductivity, which increases the resistivity of the first pole piece region, so that the short-circuit current can be reduced during acupuncture abuse, heat generation can be reduced, and heat generation of the side reaction between the region and the electrolyte can be reduced in the hot box test. In addition, because the undercoat layer and the topcoat layer are only provided at the winding tail end (i.e., local) of the positive electrode sheet, the excessive increase in the thickness of the battery cell can be avoided, thereby improving its overall safety without almost losing the energy density of the battery cell. The use of an electrolyte containing a carboxylic acid ester solvent can further improve the low temperature performance of the battery cell.

[0066] The research in the present application found that when the resistivity R of the first electrode area and the mass content A% of the carboxylic acid ester solvent in the electrolyte satisfy 8≤R / A≤80, on the one hand, it can ensure that the battery cell has a high energy density and overall safety, and on the other hand, it can further improve the low-temperature performance of the battery cell, so that the wound battery cell of the present application has good electrical performance and safety while also having good low-temperature performance.

[0067] As an example, the value of R / A can be 8, 10, 15, 25, 40, 55, 70, 80, etc., or within the range formed by any two of the above values. If the value of R / A is too small, the safety performance of the battery cell will be affected, while if the value of R / A is too large, it is not conducive to the low-temperature performance of the battery cell.

[0068] To further improve the thermal safety of the battery cell, in some embodiments, the electrolyte further includes a lithium salt and a nitrile additive. Based on the mass of the electrolyte, the mass content B% of the nitrile additive is in the range of 2% - 10%. As an example, the mass content of the nitrile additive can be 2%, 4%, 6%, 8%, 10%, etc., or within the range formed by any two of the above values. If the content of the nitrile additive is too small, the expected effect cannot be achieved, while if the content of the nitrile additive is too high, the viscosity of the electrolyte will increase, especially the viscosity at low temperatures, thereby affecting the low-temperature performance of the battery cell.

[0069] As an example, the nitrile additive can be at least one of succinonitrile, glutaronitrile, adiponitrile, trans-butenedinitrile, trans-hexenedinitrile, glycerol trinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile, 1,2,3-tris(cyanoethoxy)propane, 1,4-dicyano-2-butene.

[0070] As an example, the lithium salt can be lithium hexafluorophosphate (LiPF 6 )), lithium tetrafluoroborate (LiBF 4 ), lithium perchlorate (LiClO 4 ), lithium hexafluoroarsenate (LiAsF 6 ), lithium hexafluoroantimonate (LiSbF 6 ), lithium difluorophosphate (LiPF 2 O 2 ), 4,5-dicyano-2-trifluoromethylimidazole lithium (LiDTI), lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI), lithium nitrate (LiNO 3 ), LiN(SO 2 RF) 2 , LiN(SO 2 F)(SO 2 RF) or a combination of one or more of them, where RF = C n F 2n+1 , and n is an integer from 1 to 10.

[0071] It is found in the research of this application that when the mass content B% of the nitrile additive in the electrolyte and the resistivity R of the first electrode region satisfy 30 ≤ R / B ≤ 200, on the one hand, it can further ensure that the battery cell has a high energy density and overall safety, and on the other hand, it can also improve the low-temperature performance of the battery cell, so that the wound battery cell of this application has good electrical performance and safety while also having good low-temperature performance.

[0072] As an example, the value of R / B can be 30, 60, 90, 120, 150, 180, 200, etc. or within the range composed of any two of the above values. If the value of R / B is too small, the battery cell cannot balance thermal safety and low-temperature performance. If the value of R / B is too large, it will affect the performance of the battery cell's electrical properties.

[0073] In some embodiments, an ARC self-heat release test is performed on the positive electrode sheet, and the specific test method is as follows:

[0074] The wound battery cell of this application is charged to full capacity with a 0.5C current (cut-off at 0.02C). In the dissection room, the fully charged positive electrode sheet is disassembled. The fully charged positive electrode sheet and the electrolyte (ethylene carbonate: diethyl carbonate = 3:7, volume ratio, where the lithium salt is 1.0 mol / L LiPF 6 ) are loaded into an aluminum-plastic film and sealed to prepare a dummy battery cell, and the heating-waiting-heating mode of the ARC device is used to test until the sample thermal runaway occurs. Define the time required for the surface temperature of the first electrode region to reach 150 °C as T 1 , for example, 1500 min. The time required for the surface temperature of the second electrode region and / or the third electrode region to reach 150 °C is T 2 , for example, 1100 min, T 1 - T 2 > 300 min. This shows that adding a surface coating can significantly improve the thermal safety of the positive electrode sheet.

[0075] In some embodiments, the coating porosity P 1 of the first electrode region is 10% - 14%. As an example, P 1 can be 10%, 11%, 12%, 13%, 14%, etc. or within the range composed of any two of the above values. The coating porosity P 2 of the second electrode region is 14% - 18%. As an example, P 2 can be 14%, 15%, 16%, 17%, 18%, etc. or within the range composed of any two of the above values. The coating porosity P 3 of the third electrode region is 18% - 25%. As an example, P 3It can be 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, etc., or within the range composed of any two of the above values.

[0076] It is found in the research of this application that when the coating porosity P of the first pole piece area 1 , the coating porosity P of the second pole piece area 2 , and the coating porosity P of the third pole piece area 3 satisfy P 1 < P 2 < P 3 , this shows that, compared with the third pole piece area, due to the existence of the bottom coating in the second pole piece area, the porosity of the coating in the second pole piece area is reduced, that is, the compactness of the coating in the second pole piece area is increased, thereby effectively preventing the contact between the positive electrode current collector and the negative electrode active material layer during nail penetration abuse and improving the nail penetration safety of the battery cell; similarly, compared with the second pole piece area, since the first pole piece area includes both the bottom coating and the surface coating, the porosity of the coating in the first pole piece area is the smallest and the compactness is the highest, which can not only reduce the infiltration ability of the electrolyte in this area and reduce the heat generation of side reactions, but also better resist the nail penetration test, thereby improving the overall safety of the battery cell. Moreover, compared with the prior art of setting the bottom coating or the surface coating on the entire positive electrode sheet, in this application, only the bottom coating and the surface coating need to be set at the winding end of the positive electrode sheet, which can effectively reduce the risk of the battery cell in tests such as nail penetration and hot box, and at the same time prevent the battery cell from being too thick and affecting the volume energy density, ensuring the improvement of its overall safety without affecting the electrical performance of the battery cell. By further using an electrolyte containing a carboxylic ester solvent, the low-temperature performance of the battery cell can be further improved.

[0077] Please continue to refer to Figure 2 , in some alternative embodiments, on the same side in the thickness direction of the positive electrode sheet, the length L of the bottom coating 13 1 and the length L of the positive electrode active layer 12 2 and the length L of the surface coating 14 3 satisfy: (L 1 + L 3 ) ≤ 1.5 * L 2 . By controlling the lengths of the bottom coating and the surface coating within the above range, on the basis of ensuring good overall safety of the battery cell, the excessive increase in the thickness of the battery cell can be avoided as much as possible. By further using an electrolyte containing a carboxylic ester solvent, the battery cell has the advantages of high energy density, good safety and low-temperature performance.

[0078] It can be understood that the length directions of the positive electrode active layer, the bottom coating, and the surface coating are all consistent with the length direction of the positive electrode sheet, and the length direction of the positive electrode sheet is perpendicular to the thickness direction of the positive electrode sheet.

[0079] Specifically, in some embodiments, the length L of the bottom coating 1 is 100 mm - 2000 mm. As an example, L 1 can be, for example, 100 mm, 300 mm, 500 mm, 800 mm, 1000 mm, 1300 mm, 1500 mm, 1800 mm, 2000 mm, etc. or within the range formed by any two of the above values.

[0080] In some embodiments, the length L of the positive electrode active layer 2 is 500 mm - 3000 mm. As an example, L 2 can be, for example, 500 mm, 800 mm, 1000 mm, 1300 mm, 1500 mm, 1800 mm, 2000 mm, 2300 mm, 2500 mm, 2800 mm, 3000 mm, etc. or within the range formed by any two of the above values.

[0081] In some embodiments, the length L of the surface coating 3 is 50 mm - 500 mm. As an example, L 3 can be, for example, 50 mm, 80 mm, 100 mm, 200 mm, 300 mm, 400 mm, 500 mm, etc. or within the range formed by any two of the above values.

[0082] Compared with the hot box test, the needle penetration test has a greater requirement for the range of the test position of the battery cell, which makes the length of the bottom coating greater than that of the surface coating. On this basis, to avoid excessive increase in the thickness of the battery cell and affect its energy density, in some embodiments, the surface coating and / or the bottom coating are partially embedded in the positive electrode active layer, and the depth h of the surface coating embedded in the positive electrode active layer 1 is less than the depth h of the bottom coating embedded in the positive electrode active layer 2 . This can reduce the internal polarization of the battery cell, reduce the loss of energy density, improve its overall safety without affecting the electrical performance of the battery cell, and at the same time, when used in combination with an electrolyte containing a carboxylic acid ester solvent, the low-temperature performance of the battery cell can be further improved.

[0083] In some exemplary embodiments, in the thickness direction of the positive electrode sheet, the minimum thickness h of the surface coating 3 is 0.5 μm - 4 μm. As an example, h 3 can be 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, etc. or within the range formed by any two of the above values; the minimum thickness h of the bottom coating 4 is 2 μm - 6 μm. As an example, h 4It can be 2μm, 3μm, 4μm, 5μm, 6μm, etc. or within the range composed of any two of the above values.

[0084] The research of this application finds that the minimum thickness h of the surface coating 3 is less than or equal to the minimum thickness h of the bottom coating 4 When this is the case, the internal polarization of the battery cell can be reduced, the energy density loss can be decreased, and at the same time, the safety performance of the battery cell can be ensured. When used in combination with an electrolyte containing a carboxylic ester solvent, the low-temperature performance of the battery cell can be further improved.

[0085] Please refer to Figure 3 and Figure 4 , in some embodiments, the thickness of the surface coating is, for example, 1.98μm, 3.57μm, 3.97μm, and the thickness of the bottom coating is 2.86μm, 3.37μm, 5.21μm. From Figure 3 and Figure 4 It can be seen that the embedding depth h of the surface coating into the positive electrode active layer 1 is 0.5μm - 3μm. For example, it can be 0.5μm, 1μm, 1.5μm, 2μm, 2.5μm, 3μm, etc. or within the range composed of any two of the above values; the embedding depth h of the bottom coating into the positive electrode active layer 2 is 1μm - 3μm. For example, it can be 1μm, 1.5μm, 2μm, 2.5μm, 3μm, etc. or within the range composed of any two of the above values.

[0086] It can be understood that the "embedding depth" represents the degree of embedding of the bottom coating / surface coating into the positive electrode active layer, and is represented by the difference between the maximum thickness and the minimum thickness of the bottom coating / surface coating in the cross-sectional SEM image at the junction of the bottom coating / surface coating and the positive electrode active layer per unit area (mm 2 ).

[0087] In some embodiments, based on the mass of the bottom coating, the bottom coating includes 80% - 90% of the first inorganic particles, 0.5% - 10% of the first binder, and 0.5% - 10% of the conductive agent. The above % is the mass percentage content.

[0088] As an example, the first inorganic particles include at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide; the first binder includes polyacrylic acid (PAA), polyacrylate, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyolefins (such as polypropylene PP, polyethylene PE and other olefin copolymers), fluorinated rubber, polyimide (PI) or their derivatives; the conductive agent includes at least one of conductive carbon black (SP), acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, carbon fiber, and graphene; the metal powder includes at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, and aluminum powder.

[0089] The base coating layer adopts non-conductive or low-conductive first inorganic particles, which can ensure good insulation when the base coating layer contacts the negative electrode active material layer in the puncture test, thereby improving the puncture safety of the battery cell; at the same time, in order to ensure the electron conduction between the positive electrode active layer and the positive electrode current collector, the base coating layer needs to have a certain conductivity, so the base coating layer also includes a small amount of conductive agent.

[0090] In some embodiments, based on the mass of the top layer, the top layer comprises 90%-99% of the second inorganic particles and 1%-10% of the second binder. The above % is the mass percentage.

[0091] As an example, the second inorganic particles include at least one of silica, magnesium oxide, aluminum oxide, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide; the second binder includes at least one of polyacrylic acid (PAA), polyacrylate, styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), polyolefins (such as polypropylene PP, polyethylene PE and other olefin copolymers), fluorinated rubber, polyimide (PI) or their derivatives.

[0092] The surface coating layer uses non-conductive or low-conductive second inorganic particles, which makes the surface coating layer insulating or low-conductive, which can slow down the oxidation reaction of the electrolyte at high potential and reduce the occurrence of side reactions. At the same time, it can also increase the internal resistance of the battery cell when it is out of control and improve the thermal safety of the battery cell.

[0093] In some embodiments, the diaphragm includes a substrate, a first glue layer, a ceramic layer and a second glue layer, the first glue layer is arranged on one side surface of the substrate, the second glue layer is arranged on the surface of the substrate away from the first glue layer, and the ceramic layer is located between the substrate and the second glue layer and faces the positive electrode sheet.

[0094] The ceramic layer includes ceramic particles. In some embodiments, the median particle size Dv50 of the ceramic particles is 0.5 μm - 2 μm. As an example, it can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, etc. or within the range composed of any two of the above values. In this application, the median particle size Dv50 of the ceramic particles is obtained by testing with a laser particle size analyzer.

[0095] Using smaller ceramic particles makes the ceramic layer thinner and the heat shrinkage property of the separator better, which can prevent the short circuit of the battery cell under overheating or abnormal conditions, thus ensuring the safety of the battery. As an example, the ceramic particles include at least one of boehmite, alumina, zirconia, silica, silicon carbide, and silicon nitride.

[0096] This application's research found that when 0.5 ≤ H / Dv50 ≤ 20 is satisfied between the median particle size Dv50 of the ceramic particles and the average thickness H of the surface coating, the thermal safety of the battery cell can be further improved. At the same time, since the surface coating and the bottom coating are only provided at the winding end of the positive electrode sheet, the excessive increase in the thickness of the battery cell can be avoided, so that the overall safety can be improved on the premise of hardly losing the energy density of the battery cell. Coupled with the use of an electrolyte containing a carboxylic ester solvent, the low-temperature performance of the battery cell can be further improved. If the value of H / Dv50 is too small, it means that the particle size of the ceramic particles is too large or the thickness of the surface coating is too small, which is not conducive to the thermal safety of the battery cell; if the value of H / Dv50 is too large, it means that the particle size of the ceramic particles is too small or the thickness of the surface coating is too large, which will cause more loss of the battery energy density and is also not conducive to the improvement of the low-temperature performance.

[0097] As an example, the value of H / Dv50 can be 0.5, 1, 3, 7, 10, 13, 17, 20, etc. or within the range composed of any two of the above values.

[0098] It should be noted that the average thickness of the surface coating is obtained by measuring the thickness of the surface coating at 10 positions in the cross-sectional SEM image of the positive electrode sheet and calculating the average value.

[0099] Specifically, in some embodiments, the average thickness H of the surface coating is 1 μm - 10 μm. As an example, H can be 1 μm, 3 μm, 5 μm, 7 μm, 10 μm, etc. or within the range composed of any two of the above values.

[0100] According to the second aspect of this application, a lithium-ion secondary battery is provided, including the wound battery cell described in the first aspect of this application.

[0101] The present application will be further described in detail below in conjunction with specific embodiments, which should not be construed as limiting the scope claimed by the present application. For those embodiments and comparative examples where specific experimental steps or conditions are not indicated, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For reagents or instruments whose manufacturers are not indicated, they are all conventional reagent products that can be obtained through commercial purchase. In all the embodiments and comparative examples of the present application, the unit % represents mass percentage content.

[0102] Example 1

[0103] The first step: Prepare the bottom coating

[0104] Take alumina, conductive carbon black, carbon nanotubes and PVDF in a mass ratio of 90:2:3:5 in sequence. After mixing alumina with carbon black and carbon nanotubes evenly, then add PVDF and the solvent N-methylpyrrolidone (NMP) and stir evenly to obtain a bottom coating slurry with a solid content of 30%. Coat the bottom coating slurry on both surfaces in the thickness direction of the aluminum foil by gravure coating, and obtain a positive current collector with a partial bottom coating after drying.

[0105] The second step: Prepare the positive electrode active layer

[0106] Add conductive carbon black and carbon nanotubes to PVDF and stir evenly, then add lithium cobaltate and stir evenly to prepare a positive electrode active material slurry with a solid content of 75%. Coat the positive electrode active material slurry on both surfaces of the positive current collector with a partial bottom coating prepared in the first step, and obtain the positive electrode active layer after drying. The mass fraction of lithium cobaltate in the positive electrode active layer is 97.6%, the mass fraction of PVDF is 1.05%, and the mass fraction of conductive carbon black and carbon nanotubes is 1.35% (the mass ratio of carbon black to carbon nanotubes is 1:1).

[0107] For the sake of easy expression, the two surfaces of the positive electrode sheet are respectively defined as surface A and surface C, where the length of the positive electrode active layer on surface A is greater than the length of the positive electrode active layer on surface C.

[0108] The third step: Prepare the surface coating

[0109] Add silicon dioxide to PVDF and stir evenly, then add NMP and stir evenly to prepare a surface coating slurry with a solid content of 25%. Coat the surface coating slurry on the surface of the positive electrode active layer on surface A prepared in the second step, and obtain the positive electrode sheet after baking and rolling. The mass fraction of silicon dioxide in the surface coating is 90%, and the mass fraction of PVDF is 10%.

[0110] On the surface A of the positive electrode sheet prepared in this embodiment, the length L of the bottom coating 1 is 200 mm, and the length L of the positive electrode active layer 2 is 1500 mm, and the length L of the surface coating3 is 120 mm, satisfying (L 1 + L 3 ) ≤ 1.5 * L 2 .

[0111] Step 4: Prepare the negative electrode sheet

[0112] Mix 97.3% graphite mixed silicon-carbon negative electrode active material (containing 5% silicon-carbon), 0.5% conductive carbon black, 1.3% PVDF, and 0.9% CMC evenly, then add an appropriate amount of deionized water and disperse evenly to prepare the negative electrode active material slurry. Coat the two side surfaces of the carbon-coated copper foil with the negative electrode active material slurry, and obtain the negative electrode sheet after baking and rolling.

[0113] Step 5: Prepare the lithium-ion secondary battery

[0114] Wind the positive and negative electrode sheets after slitting and sheet-making with the separator to obtain a wound core. The wound core is subjected to encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV to obtain the lithium-ion secondary battery. Among them,

[0115] The A side of the positive electrode sheet faces the winding center of the wound core, and the C side faces away from the winding center of the wound core;

[0116] The electrolyte is a commercially available conventional electrolyte. The content of ethyl propionate in the solvent is 30%, the content of adiponitrile is 6%, and the lithium salt is LiFP 6 ;

[0117] The separator includes a first adhesive layer, a substrate layer, a ceramic layer, and a second adhesive layer stacked in sequence. The first adhesive layer is bonded to the negative electrode sheet, the second adhesive layer is bonded to the positive electrode sheet, and the particle size Dv50 of the ceramic particles in the ceramic layer is 0.7 μm.

[0118] Figure 2 This is the cross-sectional SEM image of the surface coating prepared in this example. It can be seen from the figure that part of the surface coating is embedded in the positive electrode active layer. The thickness of the surface coating at different positions can be 1.98 μm, 3.57 μm, 3.97 μm. In the unit area of the surface coating mm 2 The difference between the maximum value and the minimum value of the surface coating thickness (i.e., the depth of the surface coating embedded in the positive electrode active layer) h 1 is 1.99 μm. The average thickness H of the surface coating is 5 μm.

[0119] Figure 3 This is the cross-sectional SEM image of the bottom coating prepared in this example. It can be seen from the figure that part of the bottom coating is embedded in the positive electrode active layer. The thickness of the bottom coating at different positions can be 2.86 μm, 3.37 μm, 5.21 μm. In the unit area of the bottom coating mm 2The difference between the maximum value and the minimum value of the inner bottom coating thickness (i.e., the depth of the bottom coating embedded in the positive electrode active layer) h 2 is 2.35 μm.

[0120] Example 2

[0121] Except for the following content, the rest is the same as Example 1.

[0122] The content of ethyl propionate, a carboxylic acid ester solvent in the electrolyte, is 10%.

[0123] Example 3

[0124] Except for the following content, the rest is the same as Example 1.

[0125] The content of ethyl propionate, a carboxylic acid ester solvent in the electrolyte, is 50%.

[0126] Example 4

[0127] Except for the following content, the rest is the same as Example 1.

[0128] 30% of ethyl propionate in the electrolyte is replaced with 20% of propyl propionate.

[0129] Example 5

[0130] Except for the following content, the rest is the same as Example 1.

[0131] 30% of ethyl propionate in the electrolyte is replaced with 15% of ethyl propionate and 15% of propyl propionate.

[0132] Example 6

[0133] Except for the following content, the rest is the same as Example 1.

[0134] 30% of ethyl propionate in the electrolyte is replaced with 25% of methyl formate;

[0135] The bottom coating includes 80% of alumina, 12% of PVDF, 4% of carbon black, and 4% of carbon nanotubes.

[0136] Example 7

[0137] Except for the following content, the rest is the same as Example 1.

[0138] 30% of ethyl propionate in the electrolyte is replaced with 12.5% of methyl chloroacetate;

[0139] The bottom coating includes 99% of alumina, 0.5% of PVDF, and 0.5% of carbon nanotubes.

[0140] Example 8

[0141] The rest is the same as in Example 1 except for the following content.

[0142] The content of adiponitrile, a nitrile additive, in the electrolyte is 10%.

[0143] Example 9

[0144] The rest is the same as in Example 1 except for the following content.

[0145] The content of adiponitrile, a nitrile additive, in the electrolyte is 3%.

[0146] Example 10

[0147] The rest is the same as in Example 1 except for the following content.

[0148] 6% of adiponitrile in the electrolyte is replaced with 4% of triglyceronitrile.

[0149] Example 11

[0150] The rest is the same as in Example 1 except for the following content.

[0151] 6% of adiponitrile in the electrolyte is replaced with 5% of succinonitrile.

[0152] Example 12

[0153] The rest is the same as in Example 1 except for the following content.

[0154] 6% of adiponitrile in the electrolyte is replaced with 8% of 1,3,6 - hexanetricarbonitrile;

[0155] The bottom coating comprises 80% of alumina, 12% of PVDF, 4% of carbon black, and 4% of carbon nanotubes.

[0156] Example 13

[0157] The rest is the same as in Example 1 except for the following content.

[0158] The content of adiponitrile, a nitrile additive, in the electrolyte is 1.8%.

[0159] Example 14

[0160] The rest is the same as in Example 1 except for the following content.

[0161] The content of adiponitrile, a nitrile additive, in the electrolyte is 21%.

[0162] Example 15

[0163] The rest is the same as in Example 1 except for the following content.

[0164] The bottom coating comprises 80% alumina, 10% PVDF, 4% carbon black and 6% carbon nanotubes.

[0165] Example 16

[0166] Except for the following content, the rest is the same as in Example 1.

[0167] The bottom coating comprises 99% alumina, 0.5% PVDF and 0.5% carbon nanotubes.

[0168] Example 17

[0169] Except for the following content, the rest is the same as in Example 1.

[0170] The bottom coating comprises 95% magnesium oxide, 3% PP and 2% magnesium metal powder.

[0171] Example 18

[0172] Except for the following content, the rest is the same as in Example 1.

[0173] The bottom coating comprises 85% silica, 5% PVA and 10% carbon fiber.

[0174] Example 19

[0175] Except for the following content, the rest is the same as in Example 1.

[0176] The bottom coating comprises 80% alumina, 12% PVDF, 4% carbon black and 4% carbon nanotubes.

[0177] Example 20

[0178] Except for the following content, the rest is the same as in Example 1.

[0179] The bottom coating comprises 89.6% alumina, 0.4% PVDF, 4% carbon black and 6% carbon nanotubes.

[0180] Example 21

[0181] Except for the following content, the rest is the same as in Example 1.

[0182] The top coating comprises 99% silica and 1% PVDF.

[0183] Example 22

[0184] Except for the following content, the rest is the same as in Example 1.

[0185] The top coating comprises 95% magnesium oxide and 5% PTFE.

[0186] Example 23

[0187] Except for the following content, the rest is the same as in Example 1.

[0188] The surface coating comprises 99.5% silica and 0.5% PVDF.

[0189] Example 24

[0190] Except for the following content, the rest is the same as in Example 1.

[0191] The surface coating comprises 89% silica and 11% PVDF.

[0192] Example 25

[0193] Except for the following content, the rest is the same as in Example 1.

[0194] On the A side of the positive electrode sheet, the length L of the bottom coating 1 is 100 mm, the length L of the positive electrode active layer 2 is 500 mm, and the length L of the surface coating 3 is 50 mm, satisfying (L 1 + L 3 ) ≤ 1.5 * L 2 .

[0195] Example 26

[0196] Except for the following content, the rest is the same as in Example 1.

[0197] On the A side of the positive electrode sheet, the length L of the bottom coating 1 is 1000 mm, the length L of the positive electrode active layer 2 is 3000 mm, and the length L of the surface coating 3 is 500 mm, satisfying (L 1 + L 3 ) ≤ 1.5 * L 2 .

[0198] Example 27

[0199] Except for the following content, the rest is the same as in Example 1.

[0200] On the A side of the positive electrode sheet, the length L of the bottom coating 1 is 2000 mm, the length L of the positive electrode active layer 2 is 2200 mm, and the length L of the surface coating 3 is 270 mm, satisfying (L 1 + L 3 ) ≤ 1.5 * L 2 .

[0201] Example 28

[0202] On the A side of the positive electrode sheet, the length L of the bottom coating 1 is 1460 mm, and the length L of the positive active material layer 2 is 1500 mm, and the length L of the surface coating 3 is 800 mm, not satisfying (L 1 + L 3 ) ≤ 1.5 * L 2 .

[0203] Example 29

[0204] Except for the following content, the rest is the same as Example 1.

[0205] The Dv50 of the ceramic particles in the separator is 1.2 μm, and the average thickness H of the surface coating is 3 μm.

[0206] Example 30

[0207] Except for the following content, the rest is the same as Example 1.

[0208] The Dv50 of the ceramic particles in the separator is 0.5 μm, and the average thickness H of the surface coating is 10 μm.

[0209] Example 31

[0210] Except for the following content, the rest is the same as Example 1.

[0211] The Dv50 of the ceramic particles in the separator is 2 μm, and the average thickness H of the surface coating is 1 μm.

[0212] Example 32

[0213] Except for the following content, the rest is the same as Example 1.

[0214] The Dv50 of the ceramic particles in the separator is 2.5 μm, and the average thickness H of the surface coating is 1 μm.

[0215] Example 33

[0216] Except for the following content, the rest is the same as Example 1.

[0217] The Dv50 of the ceramic particles in the separator is 0.5 μm, and the average thickness H of the surface coating is 11 μm.

[0218] Comparative Example 1

[0219] Except for the following content, the rest is the same as Example 1.

[0220] No bottom coating and surface coating are provided on the positive electrode sheet.

[0221] Comparative Example 2

[0222] Except for the following content, the rest is the same as in Example 1.

[0223] The content of ethyl propionate, a carboxylic acid ester solvent, in the electrolyte is 55%.

[0224] Comparative Example 3

[0225] Except for the following content, the rest is the same as in Example 1.

[0226] The content of ethyl propionate, a carboxylic acid ester solvent, in the electrolyte is 7%.

[0227] Test Example

[0228] 1. Particle size test

[0229] The test was carried out using a laser particle size analyzer.

[0230] 2. Coating thickness and embedding depth test

[0231] Obtained from the SEM image of the coating cross-section.

[0232] 3. Porosity test

[0233] Along Figure 1 the dividing line between the first electrode region I and the second electrode region II and the dividing line between the second electrode region II and the third electrode region III shown in

[0234] The first electrode region was placed in a true density tester, helium was used as the medium, the pressure in the measurement chamber was gradually increased to a specified value, and then the helium expanded into the expansion chamber. The equilibrium pressures of the two processes were automatically recorded by the instrument. According to the law of conservation of mass, after calibrating the volumes of the measurement chamber and the expansion chamber with a standard ball, the difference between the two was the true volume of the electrode. Then, according to the formula porosity = (electrode volume - true electrode volume) / electrode volume, the coating porosity of the first electrode region was calculated.

[0235] Using the same method as above, the coating porosities of the second and third electrode regions were tested and calculated.

[0236] 4. ARC self-heat release test

[0237] The battery cell was charged to full capacity with a current of 0.5C (cut-off at 0.02C). In the dissection room, the fully charged positive electrode was disassembled. The fully charged positive electrode and the electrolyte (ethylene carbonate: diethyl carbonate = 3:7, volume ratio, where the lithium salt is 1.0 mol / L LiPF 6 ) were sealed in an aluminum-plastic film to prepare a dummy battery cell, and the heating-waiting-heating mode of the ARC device was used to test until the sample experienced thermal runaway.

[0238] 5. Resistivity test

[0239] The battery is discharged to the lower limit voltage, then disassembled in a drying room, the positive electrode is dried naturally in the drying room, and then the positive electrode / negative electrode is placed on a four-probe resistivity tester for testing.

[0240] 6. Energy density retention test

[0241] The prepared finished battery was charged to full power at constant current and constant voltage at 25°C, and then discharged to 3.0V at 0.5C, and the discharged capacity was recorded as the battery capacity;

[0242] The prepared finished battery was charged to 50% SOC at 25°C, and the battery thickness was tested using 600g PPG;

[0243] Calculate volume energy density (ED) = battery capacity * platform voltage / battery length / width / thickness.

[0244] Calculate the ED loss rate = (energy density of comparative example 1 - energy density of embodiment) / energy density of comparative example 1.

[0245] 7. Acupuncture test

[0246] At room temperature, discharge the battery to 3.0V at 1C, charge it to 4.50V at 0.7C constant current, and the cut-off current is 0.02C, discharge it to 3.0V at 1C, cycle it 5 times, and charge it to 4.50V at 0.7C constant current, and the cut-off current is 0.02C. Within 48 hours after the test is completed, use a 2.5mm diameter steel needle to vertically penetrate the left, middle, and right positions of the lithium-ion battery at a speed of 30mm / s. If there is no fire or explosion, it passes. Test 20 samples and observe whether each sample passes the test.

[0247] 8. Hot box test

[0248] Fully charge the battery to the upper limit voltage of 4.5V, then place the battery in an oven, heat it to the set target temperature (130℃, 132℃, 135℃ or 140℃) at a temperature rise rate of 5±2℃ / min, and keep it for 60 minutes, then the test ends; the battery is considered to have passed if it does not catch fire or explode; if the battery temperature continues to rise until it catches fire or explodes, it has failed the test. Test 10 samples and observe whether each sample passes the test.

[0249] 9. Low temperature discharge performance test

[0250] Charge the battery cell to the upper limit voltage of 4.5V when fully charged, and then discharge it at 0.2C to 3.0V. The discharged capacity is recorded as the battery capacity C1; transfer it to an incubator at -20°C, let it stand for 4 hours, and then discharge it at 0.2C to 3.0V. The discharged capacity is recorded as the battery capacity C2; the low-temperature discharge capacity retention rate = C2 / C1*100%.

[0251] The above test results are shown in Table 1 - Table 2. " / " in Table 1 indicates that this item does not exist, P 1 represents the coating porosity of the first electrode sheet area, P 2 represents the coating porosity of the second electrode sheet area, P 3 represents the coating porosity of the third electrode sheet area, T 1 is the time required for the surface temperature of the first electrode sheet area to reach 150°C, T 2 is the time required for the surface temperature of the second electrode sheet area to reach 150°C, R is the resistivity of the first electrode sheet area, A% is the content of carboxylic ester solvents in the electrolyte, and B% is the content of nitrile additives in the electrolyte.

[0252] Table 1

[0253]

[0254]

[0255] Table 2

[0256]

[0257]

[0258]

[0259] Combined with Table 1 - Table 2, it can be seen that the positive electrode sheet in Comparative Example 1 is not provided with a bottom coating and a surface coating. Therefore, the safety of the battery in Comparative Example 1 is poor and it cannot pass the 2.5mm needle puncture test and the 140°C furnace temperature test.

[0260] Compared with Comparative Example 1, the battery safety performance of all examples has been improved to varying degrees, and the ED loss rate is generally low, and the low-temperature discharge performance is good. Compared with Example 1, the positive electrode sheets in Comparative Examples 2 - 3 are also provided with a bottom coating and a surface coating, so the safety performance of the battery can be improved. However, the content of carboxylic ester solvents in the electrolyte of Comparative Example 3 is too low, which is not conducive to the improvement of the low-temperature performance of the battery. On the contrary, the content of carboxylic ester solvents in the electrolyte of Comparative Example 2 is too high. Although it can improve the low-temperature discharge performance to a certain extent, it is not that the higher the content of carboxylic ester solvents, the better the low-temperature performance. At the same time, it will also affect the thermal safety of the battery.

[0261] This shows that by providing a bottom coating and a surface coating at the winding end of the positive electrode sheet, the overall safety of the battery can be improved with a relatively small loss of the energy density of the battery cell. At the same time, by using an electrolyte containing an appropriate amount of carboxylic ester solvent, the low-temperature performance of the battery can be improved, so that the battery of the present application has both good overall safety and low-temperature performance.

[0262] Obviously, the above embodiments are merely examples given for clear illustration and are not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A wound battery cell, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet, the separator and the negative electrode sheet are stacked and wound; characterized in that: The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side surface of the positive electrode current collector, and a base coating layer and a surface coating layer are arranged at the winding tail end of the positive electrode sheet; The primer layer is located between the positive electrode current collector and the positive electrode active layer, and the primer layer includes first inorganic particles; The surface coating layer is located on the surface of the positive electrode active layer away from the positive electrode current collector, and the surface coating layer includes second inorganic particles; The electrolyte includes a carboxylate solvent, and the mass content of the carboxylate solvent is A%, based on the mass of the electrolyte, and 10≤A≤50.

2. The wound battery cell according to claim 1, characterized in that: The positive electrode sheet includes a first electrode sheet area, a second electrode sheet area and a third electrode sheet area; The third pole piece region, the second pole piece region, and the first pole piece region are sequentially arranged along the winding direction of the positive pole piece; The first pole piece region includes the positive electrode current collector and the base coating layer, the positive electrode active layer and the surface coating layer arranged on the surface of the positive electrode current collector facing the inner side of the wound battery core; The resistivity of the first pole piece region is R, 200Ω·cm≤R≤1000Ω·cm; Preferably, 8≤R / A≤80 is satisfied; Preferably, the carboxylate solvent includes at least one of methyl formate, ethyl acetate, ethyl propionate, methyl acetate, methyl butyrate, methylpropyl carbonate, propyl propionate, ethyl butyrate, methyl fluoroformate, ethyl difluoroacetate, ethyl 2-fluoropropionate, methyl fluoroacetate, methyl 4-fluorobutyrate, propyl 2-fluoropropionate, and ethyl 4-fluorobutyrate.

3. The wound battery cell according to claim 2, characterized in that: Satisfy: 500Ω·cm≤R≤800Ω·cm, and / or, 10≤R / A≤80; And / or, the electrolyte further comprises a lithium salt and a nitrile additive, and the mass content of the nitrile additive is B%, based on the mass of the electrolyte, 2≤B≤10; Preferably, 30≤R / B≤200 is satisfied; Preferably, the nitrile additive includes at least one of succinonitrile, glutaronitrile, adiponitrile, trans-butenedinitrile, trans-hexenedinitrile, glycerol trinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetrinitrile, 1,2,3-tri(cyanoethoxy)propane, and 1,4-dicyano-2-butene.

4. The wound battery cell according to claim 2, characterized in that: The second electrode area includes the positive electrode current collector and the primer layer and the positive electrode active layer disposed on both side surfaces of the positive electrode current collector; The third electrode region includes the positive electrode current collector and the positive electrode active layer disposed on both side surfaces of the positive electrode current collector; In the ARC self-heating test, the time required for the surface temperature of the first pole piece region to reach 150° C. is T1, the time required for the surface temperature of the second pole piece region and / or the third pole piece region to reach 150° C. is T2, and T1-T2>300 min; And / or, the coating porosity P1 of the first pole piece region is 10%-14%, the coating porosity P2 of the second pole piece region is 14%-18%, and the coating porosity P3 of the third pole piece region is 18%-25%, satisfying P1<P2<P3.

5. The wound battery cell according to claim 1, characterized in that: On the same side in the thickness direction of the positive electrode sheet, the length L1 of the undercoat layer, the length L2 of the positive electrode active layer, and the length L3 of the surface coat layer satisfy: (L1+L3)≤1.5*L2; Preferably, L1 is 100mm-2000mm; Preferably, L2 is 500mm-3000mm; Preferably, L3 is 50mm-500mm.

6. The wound battery cell according to claim 1, characterized in that: The surface coating layer and / or the undercoat layer are partially embedded in the positive electrode active layer, and the depth h1 of the surface coating layer embedded in the positive electrode active layer is less than the depth h2 of the undercoat layer embedded in the positive electrode active layer; Preferably, the depth h1 of the surface coating layer embedded in the positive electrode active layer is 0.5 μm-3 μm; Preferably, the depth h2 of the primer layer embedded in the positive electrode active layer is 1 μm-3 μm.

7. The wound battery cell according to claim 1, characterized in that: In the thickness direction of the positive electrode sheet, the minimum thickness h3 of the topcoat layer is 0.5 μm-4 μm, and the minimum thickness h4 of the bottomcoat layer is 2 μm-6 μm, satisfying h3≤h4.

8. The wound battery cell according to any one of claims 1 to 7, characterized in that: Based on the mass of the primer layer, the primer layer comprises 80%-99% of first inorganic particles, 0.5%-10% of a first binder, and 0.5%-10% of a conductive agent; and / or, based on the mass of the topcoat layer, the topcoat layer comprises 90%-99% of the second inorganic particles and 1%-10% of the second binder; Preferably, the first inorganic particles and / or the second inorganic particles include at least one of silicon dioxide, magnesium oxide, aluminum oxide, calcium oxide, boehmite, magnesium hydroxide, and titanium dioxide; Preferably, the first binder and / or the second binder comprises at least one of polyacrylic acid, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose, polyacrylonitrile, polyvinylidene fluoride, polyvinyl alcohol, polytetrafluoroethylene, polyolefin, fluorinated rubber, polyimide or derivatives thereof; Preferably, the conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, conductive carbon fiber, carbon nanotubes, metal powder, carbon fiber, and graphene; more preferably, the metal powder includes at least one of magnesium powder, copper powder, zirconium powder, lithium powder, calcium powder, manganese powder, sodium powder, and aluminum powder.

9. The wound battery cell according to claim 1, characterized in that: The diaphragm comprises a substrate, a first adhesive layer, a ceramic layer and a second adhesive layer, wherein the first adhesive layer is arranged on one side surface of the substrate, the second adhesive layer is arranged on a surface of the substrate away from the first adhesive layer, and the ceramic layer is located between the substrate and the second adhesive layer and faces the positive electrode sheet; The ceramic layer includes ceramic particles, and the median particle size Dv50 of the ceramic particles and the average thickness H of the surface coating layer satisfy the following relationship: 0.5≤H / Dv50≤20, H is 1 μm-10 μm, and Dv50 is 0.5 μm-2 μm; Preferably, the ceramic particles include at least one of boehmite, alumina, zirconia, silicon dioxide, silicon carbide and silicon nitride.

10. A lithium ion secondary battery, characterized in that: A wound battery cell comprising any one of claims 1 to 9.

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